Stresses On a Blade

Back in November 2018 I posted a link to a video of the deflection on a katana blade as it cuts through some rolled-up matting. [stderr]

It is always impressive to watch steel deflect. The stuff is like thick clay when it’s heated above normalizing temperature (over 1800F, roughly) but when it’s been hardened and tempered it comes alive and every work-piece has its own resonant frequency and generally it won’t bend unless you put it in a vise and use leverage, or hammer on that. I am thinking of that right now because I spent the afternoon experimenting on a little letter opener blade made from scrap, and damn is it stiff and tough. It looks like it ought to snap from hand-strength alone but trying to break it bare-handed would result in a trip to the emergency room and now is a bad time for that.

it’s practically an arrowhead (only about half ground)

There are varieties of “internet tameshigiri” that I shy away from watching. One of the worst, in my mind, are the ones where there is someone cutting and, on the other side of the target, there are a bunch of people sitting around watching. Sitting right where, you know, the end of the blade is going to come whizzing like a vengeful flying hunk of death if the sword fails during the cut. There was a tragic accident many years ago, in which a master swordsman – legitimately, a serious and very experienced person – had the blade disassemble from the handle and fly into the crowd, killing a little girl and injuring several other people. The master committed suicide some time later. It’s also one of the reasons I have trouble watching Forged In Fire – Doug Marcaida may know how to adeptly wield little whacking sticks but he handles a sword like it’s a baseball bat and it’s just scary to watch when you know that a) there are other people in the room and b) the blade was made under time pressure. Because “hurry up and make blade faster!” is exactly the way to get good results from people. Ugh. I’m embarrassed for Marcaida who, obviously, is a martial artist who understands “practice” yet has apparently declined to give more than a passing thought to sword-handling.

Walter Sorrells, another modern swordmaker, thinks about this stuff carefully and does lots of testing; I respect him and his work. At some point someone was asking him about tameshigiri and he explained that he could make a sword that cut mats incredibly well, by making it thin with very flat bevels, but it would be weak if you tried to use it to parry or if you hit the target at anything other than a perfectly correct angle. When your angle is not perfect, the blade can deflect, which makes the angle sharper, which makes it deflect more, which makes the angle sharper until the blade bends permanently or snaps. A proper katana blade ought not to snap like that because the steel at the edge is the only part of the blade that is hardened – although I’ve seen fancy-looking Japanese-style blades that have hardening patterns all over them, for appearance, which might mean that the blade has some built-in fracture-points. Keep that one away from Doug Marcaida.

Anyhow, this one is pretty intense.

As my friend Eric V.S. said, “will they begin making armor out of diet coke bottles next?”

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There are a fair number of martial artists on instagram who appear to me to be waving swords around, as if they are cheerleaders’ batons and not lethal weapons. One of the things the swordsman is attempting to achieve in iaijutsu practice is precision – exact alignment of the blade, tip stopping exactly in the right spot as if it’s nailed there, body and blade movement focused so as to maintain control while doing something that is dangerous even to the wielder. It drives me completely nuts when I see someone waving a sword around like its a toy – I think I blame the wushu schools of Chinese swordsmanship as demonstrated in Crouching Tiger, Hidden Dragon – whirling a dull blade around like it’s a weedwhacker or something. That fosters bad habits because it’s one great big bad habit.

Swordsman accident: The blade of a katana is held in place with a single (sometimes double) peg called a “meguki”. That whole mass of razor-sharp death depends on a little piece of bamboo a bit thicker than 1/4″ and about a 1″ long. The reason that it works at all is because all of the other pieces of the sword are carefully fitted to the blade so there is basically zero “play”, which means that the impact transfer from blade to handle and furniture is very even. Ideally. Cutting practice hammers even a well-made blade/handle pretty hard and you can induce wear that is more or less invisible. This is why many of the samurai had special battle handles for their swords, which were basically wrought iron, with an iron meguki: you don’t want your sword to spontaneously disassemble in a the middle of a battle. The reason the master swordsman killed himself was because he knew that the rest of his life, he would serve as an example to others of what not to do.


  1. kestrel says

    That video really is scary, plus it makes my wrist hurt to look at it.

    I like your letter opener. It looks like it will be very effective.

  2. sonofrojblake says

    “will they begin making armor out of diet coke bottles next?”

    Wrong question. It should read “will they begin making armor out of FULL diet coke bottles next?”… which wouldn’t be the worst idea in the world. The blade should cut through the plastic like it’s paper, but if it doesn’t, the coke is incompressible, so you get your cheap sword bent.

    Another possible answer : if they’re making swords or of whatever that thing’s made of, why the hell not?

  3. says

    Bottles of pop might be usable as improvised reactive armour. For those not familiar with it reactive armour is a common fit on modern armoured vehicles. It’s designed to dissipate the energy of shaped charges, and damage antitank projectiles. The most common version uses explosives sandwiched between two (usually) metal plates. When hit the explosive detonates, deforming the metal plates and disrupting the attacking round.

    I don’t know about other goods, but Ali Express is notorious to guitarist and bassists for the number of fake brand name guitars its sellers offer. You’d think it would be obvious that you can\t buy a 300 dollar Gibson Les Paul new, but people fall for them.

  4. lochaber says

    going on a bit of a tangent here, but if you go on youtube, there are countless videos of people setting up various objects and shooting them with various firearms and ammo.

    Simple water is surprisingly good at stopping rounds – I think a couple feet will effectively stop any round fired from most human-carried weapons (I’m guessing there are more powerful vehicle-mounted versions…).

    So, diving into a pool/lake/river might actually be an effective strategy if someone is firing at you. As long as they don’t have grenades…

  5. says

    going on a bit of a tangent here, but if you go on youtube, there are countless videos of people setting up various objects and shooting them with various firearms and ammo.

    … And there was that guy who held a telephone book against his chest and had his girlfriend shoot him with a desert eagle .50AE, because it’d get lots of views on youtube. That’s a video the survivor won’t be using.

    Water’s good because it’s heavy and repairs itself (sort of) faster than ballistic gelatine. But there are some things where its incompressibility makes it more dangerous – it carries shock waves well.

    I’ve seen movies where someone dives under the water and bullets go sleeting through the water around them (usually at an improbable angle) I’ve always wondered if that’s how it works but it ought to be easy enough to find out. Someone ought to suggest it to the youtubers (just, please, don’t go down there with scuba gear to film it)

  6. says

    The most common version uses explosives sandwiched between two (usually) metal plates. When hit the explosive detonates, deforming the metal plates and disrupting the attacking round.

    It’s really cool. If you watch slow-motion of a shaped charge, such as an EFP, the work is done by a focused shockwave. Reactive armor fires at an angle to the vehicle’s armor, and blows the focused shockwave apart, negating the shaped charge.

    It’s almost like homeopathy, except it works.

    Hey, did you hear of the homeopathic tank? It’s an old pickup truck sprayed down with a 1:1,000,000 tincture of Abrams M-1 main battle tank chobham armor.

  7. says

    The wikipedia entry on Chobham armor is really interesting [wik]
    Damn, I bet that stuff is ridiculously expensive.

    When I was in high school in the 70s we visited Aberdeen Proving Ground and saw an early prototype of an Abrams, back when it was still the XM-1. Later that year, they were moving one up from the factory and a piece of armor fell off (I suspect it was a test plate) on the freeway and some passing car hit it. Then, someone somehow figured out what it was and there was a great deal of hue and cry about the super-duper secret material and everyone being sworn to secrecy.

    It sounds like the material scientists really outdid themselves on that armor. Ceramic 5x harder than the steel armor is normally made of? I’m guessing it was super eutectic tungsten carbide or something nuts like that.

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